In a general sense, this invention relates to the forging of metals. More specifically, it relates to methods for reducing the amount of metal which is scrapped, before or during the forging process.
Large, forged articles are used in a variety of industrial applications. The articles can be formed from many metals and metal alloys, such as titanium, steel, and nickel-based superalloys. As an example, turbine engine components, such as turbine wheels or discs, are usually formed from superalloy materials. As another example, a medical prosthesis can be formed (e.g., by forging) from a titanium material. These types of articles are usually formed from billets, which had previously been forged from cast ingots. In some instances, the billets are up to about 2-3 meters in length, and 60 centimeters in thickness. They may weigh up to about 20,000 pounds (about 9000 kg). The cost of obtaining and processing the billets increases greatly with their size. The billets themselves are forged into the articles by various techniques, such as upset forging, hammer forging, and extrusion. Those familiar with the art understand that a billet can undergo dramatic changes in shape, grain size, and chemical homogeneity, during the forging operations.
Billets can contain a variety of melt-related defects, e.g., foreign bodies or “pipe”. For example, “hard alpha inclusions” of nitrogen, titanium or various silicates (or some combination thereof) sometimes appear in titanium billets. Similarly, a variety of defects can sometimes appear in superalloy billets. These defects, which are often introduced during the primary forming processes, can serve as initiation sites for points of weakness and potential failure of articles formed from the billet.
The defects in the billet can be detected by a variety of non-destructive techniques, which are described below. As an example, ultrasonic inspection can be employed, since the defects usually reflect at least a portion of an ultrasonic beam. Ultrasonic techniques are very useful for determining the size and location of defects.
In general practice, inspection of the billet for defects does not occur until completion of one of the primary forming processes, such as forging. If one or more defects are found at that stage, their position, size, and content are evaluated. If the defects represent significant, potential failure sites for the forged billet (and if they cannot be efficiently removed, e.g., by machining), the billets often must be discarded, or set aside for re-melting.
However, discarding a billet after it has been subjected to one or more forming operations can represent a considerable waste of time and resources. Thus, attempts may be made to machine away or otherwise remove discovered defects. Unfortunately, if the billet has already been subjected to a “final forging” operation, this may prove impractical. Having to repeat the entire forming process with a new casting can greatly increase overall manufacturing costs—especially in the case of very large billets.
With these concerns in mind, new methods for efficiently forging various types of billets would be welcome in the art. Specifically, the methods should reduce the amount of metal scrapped during the forging process. For example, the methods could reduce scrap by “rehabilitating” a greater number of billets. In other words, useful processes would eliminate defects at a relatively early stage of the overall forming process, or minimize the significance of those defects. The new methods should also not adversely affect the billets. Furthermore, the methods should be compatible with the overall forming processes, e.g., by not adding excessive costs to those processes.